Isn't the whole idea of an emergency stop stopping asap. As such by knowing the characteristics do you think 'I don't need to plant the stoppers yet because I know these babies'll grip'?
No, it's about getting an idea of stopping distances, seeing if there is any pull in a particular direction, how the ABS reacts to the new tyres and getting a feel for them.
By learning how they react it can improve your reactions as a driver in an emergency situation. Knowing if the tyres are going to grip if you turn the wheel whilst standing on the brake pedal is a start.